I list this as a duck recipe because that is the sort of heart I tend to amass in quantity. But any small heart will do: Chickens, pheasants, grouse, quail, turkeys --- even rabbits and hares. And frankly, you could make this with small pieces of a larger heart, such as venison or boar. You can also make it saucier by doubling the amount of broth. Don't skimp on the green things, even if you can't find watercress, arugula or frisee. Use those baby leaf lettuce mixes you see in the market, or spinach or whatever.
Mix the salt, cayenne, dry mustard and flour together. Dust the hearts with it. Heat the duck fat in a pan over medium-high heat.
When it's hot, drop the hearts into the pan and saute them for 3 to 4 minutes, shaking the pan and letting the little hearts roll around a bit.
Add the grated onion, Worcestershire sauce and broth into the pan and scrape up any browned bits on the bottom of the pan with a wooden spoon. Swirl everything around so the hearts get coated. Let the sauce reduce to a glaze, about 3 minutes or so over high heat. Mix in the parsley and serve with watecress and crusty bread.